Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, they are also used for sensor applications, such as in chemical vapor detection. Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity, phase, and/or amplitude of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency, amplitude, or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the SAW sensor. Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure.
An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave (SAW/SH-SAW) and amplitude plate mode (APM/SH-APM) devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers (IDTs) placed on a piezoelectric material. Surface acoustic wave devices may have a delay line, a filter or a resonator configuration. Bulk acoustic wave devices are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of mask. Based on the foregoing, it can be appreciated that acoustic wave devices, such as a surface acoustic wave resonator (SAW-R), surface acoustic wave filter (SAW-filter), surface acoustic wave delay line (SAW-DL), surface transverse wave (STW), bulk acoustic wave (BAW), can be utilized in various sensing measurement applications.
One promising application for micro-sensors involves oil filter and oil quality monitoring. Except under very unusual circumstances, oil does not “wear out”, “break down” or otherwise deteriorate to such an extent that it needs to be replaced. What happens is that it becomes contaminated with water, acids, burnt and un-burnt fuel, carbon particles and sludge so that it can no longer provide the desired degree of protection for engine components. Most oil filters in modern vehicles do not remove all the contaminants. A filter can only remove solid particles above a certain size. It cannot remove water, acids, or fuel dilution, all of which pass through the full-flow filter just as readily as the oil.
Motor oils are fortified with inhibitors that provide a remarkable stability and resistance to oxidation and deterioration. Such inhibitors also contain acid neutralizing additives that can eliminate acidity or engine corrosion. There is a limit, however, to the amount of contamination that even the best oil can neutralize, and there comes a time when the only satisfactory procedure is to drain the oil and replenish the engine with a new charge. Thus there arises the necessity for regular oil changes.
The question is now “How often should engine oil be changed?” Unfortunately there is no simple answer to this. From what we have already discussed, it will now be apparent that we change oil, not because it has deteriorated, but because it has become contaminated with various harmful substances, and the greater the rate at which these enter the oil, the sooner an oil change will be necessary.
The things that influence this include engine condition and method of operation. A vehicle that is used mainly for short distance stop-start running will require more frequent oil changes than one used for regular long distance traveling, and a worn engine with leaky piston rings will contaminate the oil quicker than a new engine in good mechanical condition.
It should also be in mind that a high performance product (more additives) can handle more contaminate than other products, and hence longer oil change periods can be justified. As a final comment on this subject, it is worth mentioning that it is normal for oil to darken in service. This is not an indication that the oil has deteriorated. It shows that it is picking up its load of contaminates and keeping then in suspension, where they can do no harm, and where they can be removed from the engine when the oil is changed.
Motor oil must perform two primary functions. It must lubricate the engine and it also has to serve as a collector of contamination. The contamination comes from the engine combustion chambers where the gasoline is burned to produce powder. There are two different types of fuel combustion in engines: efficient combustion or clean burning; and inefficient combustion or dirty burning.
When dirty combustion occurs in engines, soot isn't the only thing formed. Sticky, gummy products, which oil chemists call resins, and lead oxyhalides. Small quantities of acidic combustion products are also formed. Last, but by no means least, is water. For every gallon of gasoline burned, a little over one gallon of water may be formed, believe it or not! So during the burning of gasoline in engines, we have a potential problem with soot, resins, acids, and water which are formed. If these combustion products work down past the pistons to get into the crankcase oil, then we do have the problem of dirty, contaminated oil. If the oil is allowed to become too dirty and contaminated, sludge deposits will form to cause plugged piston rings, oil pump screens and oil filters. Engine wear and even engine damage can then result.
When engine is cold, the dirty combustion occurs, and the contamination work down past the pistons and get into the oil. An engine used in typical city driving with a lot of short runs, stopping and starting, seldom gets a chance to warm up thoroughly and operates at its lowest efficiency, which means dirty combustion. The cold cylinder walls of the engine act as condensers for the soot, resins, water and unburned gasoline. These are washed down past the pistons into the crankcase oil. An engine has to have a good, steady run before it is thoroughly warmed up, and a considerable longer time in winter. The first few minutes after each cold engine starts is the hardest on the oil.
After an engine has been run long enough to get thoroughly warmed up, it's then operating at its best efficiency, which means clean combustion and a minimum of combustion soot and contaminates. Furthermore, the hot cylinder walls no longer act as condensers, so the contaminates are minimized and don't work down past the pistons into the crankcase oil.
A truck, bus or passenger car driven at highway speed on a length trip is an example of perhaps the easiest job to lubricate and the least demanding on an oil of good quality. The really tough lubricating job is the engine, which experiences only short runs with lots of stops and starts, especially in cold weather. The worst conditions for both the engine and the oil are the very conditions under which the great majority of passenger cars are used most of the time.
Diesel engines are particularly hard on oil because of oxidation from acidic combustion. As the oil wears, it oxidizes and undergoes a slow build-up of total acids number (TAN). A pH sensor is capable of direct measurement of TAN and an indirect measurement of total base number (TBN), providing an early warning of oil degradation due to oxidation and excess of water. The acids and water build-up is also related to the viscosity of the oil.
Knowing the condition of oil in the field would obviously be extremely beneficial information to truck fleet maintenance managers and maintenance personnel. A permanently installed oil quality sensor system can deliver the above information.
Currently, fleets that perform analysis on their lubes utilize complete laboratory oil analysis. Primarily due to the cost of laboratory analysis, these tests are only performed on a routine basis, i.e. monthly or at each oil drain interval. Laboratory oil analysis serves two basic functions. The first function is to monitor the condition of the lube oil. Lube oil within a health engine degrades at a slow rate with normal use. Therefore, lab analysis can give us forewarning and allows us to schedule routine oil drains. Complete lab analysis is very effective in accomplishing this goal and first function.
However, it is at the second function where lab analysis falls a little short and that is giving sufficient warning as to failures such as, coolant leaks and stress related metal failures. We normally sample our equipment on a monthly basis and while this is a sufficient interval to safely monitor the lube condition, many times this frequency is not sufficient in detecting engine problems. After all, analysis is used to detect the “Problem” before “Failure” and “Downtime” can occur.
An example of this situation is as follows: A company samples their equipment on a monthly basis. On the first day of the month a sample of the used oil is taken and sent to the lab for analysis. On the second day, unknown to the maintenance personnel and the oil lab, a coolant leak develops within the engine. The next-scheduled time for another complete laboratory analysis sample to be taken is twenty-nine days away. Within the next several days, the coolant leak degrades the oil within the engine to the point that it causes wear to occur to bearings and other parts of the engine.
Somewhere between the seventh and the tenth day the operator receives the results from the lab sample taken on the first day of the month. These results were taken before the problem occurred and shows no problems within the engine and that the oil is suitable for further use. Two days after receiving this report, the operator notices that the oil is becoming cloudy and that the engine is making a little steam. The routine monthly sampling of the used oil was not effective in achieving its goal.
The need is immense for a permanently installed sensor device that can determine the condition of the lube and equipment which can be used on a more frequent basis than complete laboratory analysis sampling. This need can be met by the use of the disclosure here.
The permanently installed oil sensor could be used on oil within many different types of equipment such as, gasoline engines, diesel engines, natural gas engines, hydraulic systems, transmissions, compressors, turbines, and more. With monthly laboratory analysis, one only has 12 chances a year to catch a problem. Using the permanently installed oil sensor system on a real time basis, one can increase their chances of detecting an engine oil problem.
The permanently installed oil sensor system will prove to be an effective method of monitoring and determining the condition of both lube and equipment. The sensor system monitors the total amount of contamination present within the lube oil by measuring the viscosity and TAN of the oil. Although complete laboratory analysis delivers a more detailed analysis of the oil, this sensor unit is highly efficient in determining whether the oil and equipment is in normal operating condition. When a problem with the equipment occurs, the unit easily detects this problem by detecting the elevated TAN and viscosity of the oil due to the excess amount of contamination present within the lube oil. The permanently installed oil sensor system will be used as a simple monitoring tool to let the driver or maintenance personnel know whether the equipment is within a “Normal” or “Abnormal” operating condition.
Low temperature start-ability, fuel economy, thinning or thickening effects at high and/or low temperatures, along with lubricity and oil film thickness in running automotive engines are all dependent upon viscosity. Frequency changes in viscosity have been utilized in conventional oil detection systems. The frequency changes caused by small changes in viscosity of highly viscous liquids, however, are very small. Because of the highly viscous loading, the signal from a sensor oscillator is very “noisy” and the accuracy of such measurement systems is very poor. Moreover, such oscillators may cease oscillation due to the loss of the inductive properties of the resonator.
TAN is a property typically associated with industrial oils. It is defined as the amount of acid and acid-like material in the oil. Oxidation and nitration resins make up the majority of this material. As the oil is used, acidic components build up in the lubricant causing the TAN number to increase. A high TAN number represents the potential for accelerated rust, corrosion and oxidation and is a signal that the oil should be replaced. Critical TAN numbers are dependant on oil type.
There is a need to provide a sensor system which can be utilized to monitor, in a sensitive manner, the etching effects of etchants, such as acids contained in oils. There is also a need to provide a sensor system which can monitor corrosion or degradation of engines or other devices caused by exposure to such etchants. It is believed that acoustic wave sensors may well be suited for such monitoring as indicated by the embodiments described herein.
One of the problems with acoustic wave devices utilized in oil monitoring applications is that such devices are susceptible to attack by acids and other analytes present in the oil. When an etch rate sensor, for example, utilized oil monitoring applications is exposed to oil and/or acids, the acids tend to attack the sensor electrodes, thereby reducing the life of the sensor. It is believed that an improved etch rate sensor design and configuration is required to overcome these problems. Such a device is described in greater detail herein.